Quality of Service

It is desirable to maintain quality of service (QoS) on traffic destined for nodes on a mobile router's mobile networks. In Mobile IP, the Home Agent copies the ToS (type of service) bits from the original IP header into the new Mobile IP tunnel header before forwarding the packet to the Mobile Node/mobile router. Theoretically, this should allow the QoS on the original packet to be maintained as the packet traverses the Mobile IP tunnel. However, theory is not always reality. In reality, the encapsulated packet looks different from the original packet and, thus, can cause transit routers to change the ToS/QoS bits in the tunnel header.

Here is a case and point: Consider a network in which voice traffic receives the highest priority, as shown in Figure 7-17. When such a voice packet arrives at the Home Agent and is forwarded to the Mobile IP tunnel interface, the ToS/QoS bits are copied into the encapsulated packet before being forwarded to the mobile router. But, unsuspecting transit routers can interpret the packets as IP-in-IP packets instead of the high-priority voice packets that they really are and, thus, can modify the ToS/QoS bits. Yes, it sounds a bit rude, but this is the reality. Moreover, during the encapsulation process, the Home Agent loses information that it needs to properly carry out QoS features that require flow information on the outbound interface. If only there were a way to preserve the QoS credentials across the Mobile IP tunnel…

Figure 7-17. Integration of QoS and Mobile IP

Because Mobile IP tunnels are created dynamically, QoS interface-level configuration on the tunnel interface is not feasible. Instead, tunnel interface templates, as shown in Figure 7-18, add the QoS features on the dynamic tunnels established between the Home Agent and FA, and the dynamic tunnels generated between the Home Agent and mobile router.

Figure 7-18. Tunnel Template

NOTE

A tunnel template allows attributes to be applied to a particular tunnel when it is created. Essentially, a tunnel template is created, and a mapping is established with the tunnel template and a particular type of tunnel that says, "When this type of tunnel is created, apply this tunnel template to the tunnel."

In this manner, QoS preclassification (the qos-preclassify command) can be applied to the tunnel, allowing the original packet header to classify the packet on the outbound interface instead of the Mobile IP tunnel header. It lets the router peek into the original packet and use the QoS credentials of that packet, allowing certain traffic types to receive preferential treatment over the Mobile IP tunnel. The feature is applied dynamically to the Home Agent mobile router tunnel when it is created on the mobile router.

Example 7-2 shows how the tunnel template can be applied on the mobile router and Home Agent.

Example 7-2. Tunnel Template Configuration on the Mobile Router

 interface Tunnel50          no ip address !Turn on Qos Pre-Clasification          qos pre-classify ! ip mobile router !Tunnel template to use for Mobile IP Tunnels          template Tunnel50 Tunnel Template Configuration on Home Agent: interface Tunnel100          no ip address !Turn on Qos Pre-Clasification          qos pre-classify ! !Define Mobile Node 65.1.1.1 as a mobile router ip mobile mobile-networks 65.1.1.1 !Define template to use when creating tunnel to MR          template Tunnel100